Fast-growing forest plantations are gaining momentum around the world as human demand for wood continues to rise.

The cultivation of short-rotation tree species on non-forest land is needed for both to meet the growing demand for woody biomass for the future sustainable economy and to mitigate the effects of climate change.

In northern Europe, a common tree species used for plantations of this kind is the hybrid Populus x wettsteinii, a cross between the common aspen and the American aspen.

Under normal management, hybrid aspen is harvested every 25 years, explains Jürgen Aosaar, associate professor of forestry at the Estonian University of Life Sciences.

However, such quick and easy hybrid forest plantations are often seen as a trade-off between climate benefits and low biodiversity. Moreover, very little is known about the soil fauna in these new hybrid stands.

A team of researchers from the University of Life Sciences (EMÜ) and the University of Tartu (TU), led by Elisabeth Rähn, a doctoral student in forestry, recently found that 20-year-old hybrid aspen stands planted on former agricultural soils and European aspen stands growing on native forest soils have similar numbers of fungal species in their soils, but these are very distinct.

The researchers studied sites of hybrid aspen stands that are just under 20 years old, i.e., before their first cycle of clear-cutting, and native aspen stands of three age classes: 8–29, 30-55, and 65–131 years old.

Ectomycorrhizal fungi, or fungal species that form a symbiosis with trees, were more abundant in natural aspen soils.

The richness of saprotrophic fungi – the largest group of fungi that play a vital role in decomposing complex organic matter – was similar between native forest and plantation sites.

There were also several fungal families with similar abundance and species diversity in both hybrid plantation and woodland soils. "Several species of forest fungi can colonize hybrid aspen plantations, so the species richness of several fungal families was similar in forest and plantation at the end of the first harvest of hybrid aspen. In this way, plantations have a conservation value by supporting the spread of those type of fungi," Aosaar explains.

However, in general the fungal communities of native forests and plantations were very different, the study found. Also, the natural forest sites were somewhat more diverse than the relatively homogeneous plantations.

The researchers discovered that edaphic factors – soil properties that result from natural or human-made events – are more important for describing fungal communities in native aspen forests and in hybrid aspen plantations than age, forest thinning practices, or past land use. The soil pH was particularly singled out as the best explanatory variable to describe soil fungal communities in hybrid plantations.

So although first-generation hybrid aspen plantations and native common aspen forests are similar in overall fungal diversity, their composition is strikingly different. The team concluded, that hybrid plantations can be used to reduce felling pressure on native stands; but the knowledge so far is still insufficient to say that plantations could replace native forests from the soil biodiversity perspective.

In the hybrid stands, the species richness was mainly affected by the greater diversity of smaller trees that naturally grew under the large trees.

As a next step, Rähn wants to study soil fungi during the next harvest cycles of hybrid aspen plantations. This would help to understand the dynamics and relationships of fungal communities and individual species in an evolving environment.

The hybrid trees growing on former farmland were just under 20 years old, i.e. first-generation forests that had not yet been clear-cut. In total, the researchers included 19 stands of common aspen in native forest lands and 20 stands of hybrid aspen in the study.

The study was published in the journal Frontiers in Microbiology.

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